This invention generally relates to composite materials. More particularly, this invention relates to insulated magnetic particles. Even more particularly, this invention is related to electrically insulating coatings on ferromagnetic particles and compositions containing such coated ferromagnetic particles.
Magnetic materials fall generally into two categories: hard magnetic materials (that may be permanently magnetized) and soft magnetic materials (whose magnetization can be reversed). Iron-based magnetic (ferromagnetic) powders are often used as a component in soft magnetic materials.
Magnetic permeability and core loss are important characterizing properties of soft magnetic materials. Magnetic permeability measures the ease with which a magnetic substance may be magnetized and indicates the ability of the material to carry magnetic flux. Core loss measures the energy loss when a magnetic device is exposed to a time varying field. Core loss can be divided into two main categories: hysteresis loss and eddy current loss. Hysteresis loss measures the energy needed to overcome the retained magnetic forces in the magnetic core. Eddy current loss results from the flow of electric currents within the magnetic core induced by the changing magnetic flux.
Many electromagnetic devices contain a soft magnetic material made from laminated structures. Laminated structures typically comprise stacked thin sheets which are oriented parallel to the expected magnetic field. The sheets may often be coated to provide insulation and prevent current from circulating between the sheets. Unfortunately, the thicker this insulation layer, the lower the laminate stacking factor will be. And low stacking factors can result in reduced average magnetic permeability in the structure. As well, fabricating three-dimensional articles using laminated structures can be expensive and complex. Further, laminated structures experience large core losses at higher frequencies and can be acoustically noisy as the laminated sheets often vibrate.
Sintered or coated ferromagnetic powders have been proposed as an alternative for laminated structures in magnetic devices (or articles). These ferromagnetic powders generally allow greater variation in the geometry and avoid the manufacturing burdens resulting from laminated structures. However, articles made with sintered ferromagnetic powders exhibit high core losses and typically have restricted end-uses. Using coated ferromagnetic powders in articles, however, is a more viable alternative. The coating provides an electrical insulation for the individual ferromagnetic particles and can reduce eddy current losses. The coating can also serve as a binder or a molding lubricant in certain instances.
Various methods have been used to make magnetic articles containing coated ferromagnetic powders, including different types of coating materials and coating methods. Inorganic coating materials such as iron phosphate, iron chromate, iron oxides and boron nitride have been used. Similarly, organic coating materials have been used. Double-coated ferromagnetic powders have also been used. Polymeric materials such as polyamides, polyimides and polysulfones have been used as one coating material for ferromagnetic powders. The polymeric coating not only insulates the powder particles from one another, but also can help bind the particles together during compaction when making the magnetic article.
The magnetic properties of magnetic articles containing polymeric-coated ferromagnetic materials, however, do not allow widespread use of these materials. In particular, these materials suffer from low temperature properties of polymers that limit the high temperature annealing process that can be carried out. Instead, low-temperature annealing processes must be used that are not able to remove the cold work resulting from compaction fully, adversely affecting the permeability and losses of the magnetic articles.